1. Field of the Invention
This invention relates generally to variable frequency AC drive units and more specifically to a control system for an AC motor comprising parallel integrated variable frequency drive units.
2. Description of the Prior Art
Alternating current, or AC, motors provide much of the motive force for industry. AC motors come in a variety of styles and horsepower ratings, from fractional horsepower ratings to multiple thousands of horsepower. Control of industrial AC motors can be quite complex and typically involves more than merely starting and stopping the motor. An AC motor control system typically has to adjust the speed and torque of the motor(s) during normal operation and start-stop cycles.
AC motors may be classed as low, medium and high voltage. Medium-voltage motors consume voltages from about 400V up to about 15 kV and generate power levels up to about 20,000 Hp. For AC motor applications above about 400 Hp, it is typical in the industry to engineer a modular control system for the specific application at hand. One engineering technique used for these high end systems involves paralleling specially designed power modules (such as, rectifier, bus and inverter sections). Paralleling power modules allows flexibility in designing drive systems using a small number of available modules. A control interface specifically designed to interface with parallel power modules is used in such systems Parallel power module control systems for medium-voltage motors are inherently pensive, owing to the amount of design engineering required to configure the system, the need for many system components and the number of system interconnections (such as, wires, bus bars etc.). This design approach for high power, medium voltage control systems leads to high cost. The sophisticated and technical nature of these medium-voltage, high power control systems restrict their use to highly specialized applications.
In contrast, low voltage AC motors typically consume between about 240V and 600V and generate power levels up to about 800 Hp. Control systems for these low-voltage motors have become compact, integrated, and are produced in reasonably high volume. These “integrated” control systems are inherently less expensive than the complex, engineered systems previously mentioned.